Carbonate re-crystallization in soil revealed by 14C labeling: Experiment, model and significance for paleo-environmental reconstructions

A high CO2 concentration in soil resulting from microbial and root respiration is the main factor controlling the dissolution of primary (lithogenic, geogenic) carbonates and the formation of secondary (pedogenic) carbonates. Although several estimations of soil age and many paleo-environmental reconstructions are based on the radiocarbon age and/or δ13C of secondary carbonates, many assumptions are difficult to check experimentally because of long-term CaCO3 re-crystallization processes. In the present study we used the isotopic exchange between primary carbonates of loess and 14C respired from the rhizosphere of wheat that was artificially labeled in a 14CO2 atmosphere under controlled conditions. An ascending number of 14CO2 pulses (1 … 4) showed a linear increase of rhizosphere 14C recovered in the CaCO3 of loess. Based on this connection, the initial re-crystallization rates of loess carbonate were calculated by linear regression: for loess containing 27% CaCO3, the initial rate of carbonate re-crystallization was 0.000029 day−1. Subsequently, using linear and exponential approaches with different lengths of growing season, we extrapolate the observed CaCO3 re-crystallization on longer time periods. The calculations show that at least 100 years, but probably between 400 and 2000 years, are necessary for full (99%) re-crystallization of the CaCO3 of loess. We suggest a general equation for calculating the remaining not re-crystallized CaCO3 depending on time of soil formation (t): %CaCO3 (t) = 100·exp(−t·0.00078·Growing-Season-Length/365/initial-CaCO3-percentage). Different approaches for calculating the period of secondary carbonate re-crystallization are discussed and compared with literature data. We conclude that despite the high analytical precision of radiocarbon dating and δ13C mass spectrometry of secondary carbonates (used, e.g. for paleo-environmental reconstructions), the methodological resolution cannot be better than the periods necessary for CaCO3 re-crystallization.